Technical Paper. C. Wöhrmeyer* 1 ; J.M. Auvray 1 ; B. Li 1 ; H. Fryda 1 ; M. Szepizdyn 1 ; D. Pörzgen 2 ; N. Li 3 ; W. Yan 3.

Size: px
Start display at page:

Download "Technical Paper. C. Wöhrmeyer* 1 ; J.M. Auvray 1 ; B. Li 1 ; H. Fryda 1 ; M. Szepizdyn 1 ; D. Pörzgen 2 ; N. Li 3 ; W. Yan 3."

Transcription

1 Page : 1/9 Novel Calcium Magnesium Aluminate Bonded Castables For Steel And Foundry Ladles C. Wöhrmeyer* 1 ; J.M. Auvray 1 ; B. Li 1 ; H. Fryda 1 ; M. Szepizdyn 1 ; D. Pörzgen 2 ; N. Li 3 ; W. Yan 3. 1 SA, France; 2 Beck u. Kaltheuner GmbH & Co. KG, Germany; 3 Wuhan University of Science & Technology, China Presented at UNITECR, Victoria, Canada, September 2013

2 Page : 2/9 ABSTRACT Extensive laboratory trials have demonstrated that ladle castables bonded with the novel calcium magnesium aluminate binder (CMA) have excellent corrosion and penetration resistance. The micro spinel inside CMA facilitates the design of new castable microstructures to improve penetration and corrosion resistance. However, inside steel and foundry ladles, several destructive phenomena occur simultaneously. Slag penetration, chemical dissolution, mechanical abrasion, and thermo-mechanical phenomena happen at the same time which is difficult to simulate in classical laboratory tests. Therefore this paper investigates how CMA based castables behave under real conditions inside steel and foundry ladles. Microstructure investigations of samples taken from ladle linings give an indepth understanding of the mechanisms that lead to the improved performance of CMA based ladle castables.

3 Page : 3/9 1 Introduction Alumina-Magnesia (A-M) and Alumina-Spinel (A-MA) based monolithics bonded with Calcium Aluminate Cement (CAC) are widely used in steel ladles as working linings and functional products e.g. gas purgers, top lances, well blocks etc. Due to their excellent thermo-mechanical and thermo-chemical properties they are often used when carbon- or silica-containing bricks cannot be used in ladle linings for metallurgical reasons. With the A-M and A-MA monolithics a large range of different installation technologies can be chosen to achieve a joint-free refractory lining. The refractory consumption per ton of steel can be minimized as only the worn part of the ladle lining need to be re-lined after a certain number of heats while the non-corroded part of the wall can remain in place. The re-lining can be done by casting, by classical dry-gunning or by more advanced technologies like shotcreting. The A-MA monolithics contain typically coarse alumina aggregates, pre-formed spinel in the medium and fine fractions and CAC as binder. The spinel is introduced into the castable to enhance the slag resistance [1-2]. The CAC creates sufficient early hardening of gunning and casting materials even at low ambient temperatures [3]. The rapid hydraulic bond creates the strength that is necessary to withstand the mechanical stresses during heating, transportation and lifting of the empty and steel-filled ladle. A-M based monolithics form spinel in-situ during the first ladle heat-up to service temperatures. The primary crystal size of the formed spinel is typically much smaller than the pre-formed pure spinel powder. The smaller spinel size further enhances the slag resistance compared to the coarser primary crystals of the pre-formed spinel in A-MA castables. However, the in-situ spinel formation is accompanied by a strong increase in volume, measurable as positive Permanent Linear Change (PLC). In order to prevent excessive PLC, small quantities of silica fume are often added [4]. At the same time the silica addition reduces the sensitivity of Magnesia to hydration [5]. MgO hydration would lead to a higher water demand, higher dosage rate of deflocculant and early stiffening of the castable. During the dry-out process, MgO could hydrate with the liberated steam. Crack formation due to the high internal pressure created by the late formation of Mg(OH) 2 inside the hardened monolith could be a consequence. However, the obvious disadvantage of the silica addition is that the refractoriness is significantly reduced. Table I: Mineralogy and particle size of castable matrix components. (*stoichiometric spinel inside CMA 72, Al 2 O 3 -overstoichiometric MA in pre-syntesized spinel; **spinel inside CMA: d50 = 2-3 µm) The novel Calcium Magnesium Aluminate binder (CMA) comprises hydraulic calcium aluminates and a large quantity of MA spinel crystals (Tab. I) with a size very similar to those formed in A-M castables [6]. Applied in A-M castables, it allows reduction of both, the amount of free Magnesia and silica fume. This enhances the thermo-mechanical properties, the penetration and corrosion resistance.this has been demonstrated by extensive laboratory trials [5,6].

4 Page : 4/9 The difference in corrosion resistance has been explained with the change in the castable microstructure when CMA is used. The improvement of the corrosion resistance has also been confirmed for A-MA castables where the smallest fraction of the pre-synthesized spinel and the CAC has been replaced by CMA. Other studies have shown that CMA can also be used to replace CAC and reactive alumina which leads to an increase of total spinel content in the castable without negative impact on neither rheology nor strength [5]. With this formulation concept the higher amount of spinel in the matrix can further increase the penetration and wear resistance. Other trials have shown that by using CMA, recipes can be formulated with a very low CaO content in the castable. Together with the high amount of micro spinel it can lead to a further optimization of the thermomechanical and thermo-chemical properties. A superior corrosion resistance of CMA-based castables has also been found in contact with silica- and alkali-rich coal ashes that occur in coal gasifier applications [7]. However, with laboratory tests thermomechanical and thermo-chemical effects are tested separately with dedicated test methods. But during the ladle application all the wear effects happen at the same time. The impact of these simultaneous effects is very difficult to simulate in small laboratory tests. Therefore industrial trials in secondary steel ladles and in steel foundry ladles have been conducted with CMA based monolithics to verify if the principal wear mechanisms concluded from laboratory trials can be confirmed in full-scale industrial applications. Table II: Chemistry of castables and slag. Al2O3 MgO CaO SiO2 Fe2O3 MnO Cr2O3 CCAC >91 <5.5 <1.5 <0.3 <0.3 <0.1 <0.1 CCMA >91 <5.5 <1.5 <0.3 <0.3 <0.1 <0.1 Slag CCMA CCAC Fig. 1 XRD Diffractogram of castables. 2 INDUSTRIAL APPLICATION OF CMA Two alumina spinel castables with very similar chemical (Tab. II) and mineralogical compositions (Fig. 1) have been chosen for industrial tests. The reference castable CCAC contains a small amount of CAC (SECAR 71) and a high amount of pre-formed spinel powder. Contrary to that, the new castable CCMA uses a high amount of CMA (CMA 72) together with a small amount of the pre formed spinel powder. The total amount of spinel and calcium aluminate in both castables is the same but the distribution of spinel and calcium aluminate is quite different due to the presence of very small spinel crystals inside the CMA binder. Two 8t steel foundry ladles were prepared, one with castable CCAC as reference and the second one with newly designed CCMA. In both cases the whole ladle wall is lined with either CCAC or CCMA, including the slag zone.

5 Page : 5/9 The typical slag composition can be seen in Tab. II. Tapping temperature was around 1700 C for the first 100 heats followed by a low tapping temperature of 1480 C for another 150 heats. After cooling down to ambient temperature, samples have been taken from different parts of the ladle linings as indicated in Fig. 2. Fig. 3 Sample P5 of CCMA from ladle bottom. 4 Results and discussions Fig. 2 8t Steel foundry ladles with refractory sampling points. 3 Analytical Methods The samples have been cross-sectioned perpendicularly to the slag-refractory interface. Microstructures of these specimens have been observed by scanning electron microscopy (SEM). The composition of complex spinel and glass phases inside the microstructure has been obtained by calibration with EDAX ZAF quantification (standardless). The penetration depths in each sample have been measured by pixel counting on SEM images. Phase analyses have been carried out by X-ray diffractometer. Pore size distribution has been obtained by a microscopic measurement method through an optical microscope [8]. A sample of CCMA taken from the bottom of the ladle shows no penetration of slag or metal (Fig. 3). A large part of the original bottom thickness is still present. Only a less than 1 mm thin layer of dark slag is visible at the surface, but no macro-cracks. SEM analyses of samples from the non-slagpenetrated zone of the castables (P4, and cold sides of P1, P2) show that the spinel within the matrix of CCMA is more homogeneously distributed and finer than in castable CCAC (Fig. 4, 5). The matrix in CCMA consists of more strongly interconnected spinel, corundum, and calcium aluminates (Tab. III, IV) which creates a stronger bond. The pore sizes in CCMA are significantly smaller. A large quantity of pores has sub-micron diameters. This creates a strong protection against direct slag penetration into the matrix of CCMA. CCAC contains more macro-pores (Fig. 6) which are more easily accessible by slag. The interfaces between penetrated reaction layer and non-penetrated zones are shown in Fig. 7 and 8 together with the points of EDAX analyses which are summarized in Tab. V and VI. The slag penetrated zone in CCMA is much thinner than in CCAC. In both cases low melting phases fill the spaces between Corundum, CA6 and MA-spinel.

6 Technical Paper Page : 6/9 In castable CCMA, the average depth of reaction layer is about 100μm, only (Fig. 10). In some places there is a crack at the depth of μm, but the crack is not interconnected. Fig. 4 8t steel foundry ladles with refractory sampling points. Tab. III: EDAX analyses of CCAC (see Fig.4). Fig. 5 CCMA non-penetrated zone of P2. Tab. IV: EDAX analyses of CCMA (see Fig.5). The chemistry of the low melting phases is the result of the degree of reaction between the penetrated slag and the refractory components in the matrix. In case of CCAC the low melting phases have chemistries with oxides mainly from the system CaO-Al2O3-SiO2 and MgOAl2O3-SiO2-Fe2O3. In CCMA also some CaOMgO-Al2O3-SiO2-Fe2O3 and Na2O-Al2O3-SiO2 compositions have been found. The MA spinel in both castables is significantly enriched with Fe2O3. The transfer of Fe2O3 from liquid slag into solid spinel increases the viscosity of slag which slows down further penetration and diffusion of oxides from slags into the matrix. In castable CCAC, the average depth of the reaction layer is about 1050μm, and an interconnected crack exists at the depth of μm (Fig. 9).

7 Page : 7/9 Fig. 6 Pore size distribution. CCMA has an obviously better resistance against the wear phenomena in the ladle. The thicker penetrated layer in castable CCAC results in deeper cracks within the structure of the castable with the higher risk of spalling of this thick layer. Cleaning of the refractory surface becomes less problematical with CCMA as only a small amount of slag sticks to the surface. This makes re-lining of steel ladles faster and more economical. Fig. 11 shows another recent example of a CMA based castable that had been installed in a 190t steel ladle just below the MgO-C slag line. The sample has been taken after 30 heats when the slag line had to be changed. No significant slag penetration has been observed (<1mm) and very little slag was adhering to the surface. Microstructure analyses of this sample are ongoing. Fig. 7 Interface between reaction layer and non-penetrated area of castable CCAC. Fig. 8 Interface between reaction layer and non-penetrated area of castable CCMA.

8 Page : 8/9 Tab. V: EDAX analyses in CCAC at points indicated in Fig. 7. Tab. VI: EDAX analyses in CCMA at points indicated in Fig. 8. Fig. 9 Reaction layer at surface of CCAC.

9 Page : 9/9 5 Summary and Conclusion Industrial applications of CMA-bonded monolithics in steel and steel foundry ladles have confirmed the laboratory results with respect to the improvement of penetration and corrosion resistance. Furthermore the positive impact of penetration resistance on thermo-mechanical effects becomes obvious as cracks were much reduced with castable CCMA. CMA provides to the matrix microcrystalline and homogeneously distributed spinel when applied in castables, dry- and wet gunning mixes. The spinel creates an interconnected microstructure with reduced pore diameters that are less susceptible for slag penetration. The very fine spinel network increases the efficiency to uptake iron oxide into the spinel structure. It reduces iron oxide in the slag which stiffens the slag in the pores and creates a stronger barrier against deeper slag penetration. The longer life time combined with cleaner ladle surfaces improves the ladle management and overall cost structure. Fig. 10 Reaction layer at surface of CCMA. - Slag-penetrated layer: <1mm - Slag adhesion on surface: 1 mm Fig. 11 CMA-bonded castable from a 190t steel ladle after 30 heats. 6 References [1] T. Yamamura et al.: Development of aluminaspinel castables for steel ladles. Taikabutsu 42, 8 pp (1990). [2] S. Asano et al.: Mechanism of slag penetration in alumina-spinel castable for steel ladle: Taikabutsu 43, 4, pp (1991). [3] C. Parr et al.: Out of the mould and into the fire. Unitecr2001, Cancun, Mexico (2001). [4] D. Schmidtmeier, A. Buhr, R. Sadi, F. M.A.L. Braulio et al.: Expansion behaviour of cement bonded alumina-magnesia castables. American Ceramic Society Bulletin, Vol. 86, No. 12, pp (2017). [5] C. Wöhrmeyer et al.: New calcium magnesium aluminate for corrosion resistant castables. Proc. 6th Int. Symposium on Refractories, Zhengzhou, China, pp (2012). [6] C. Parr, C. Revais, N. Bunt, D. Jones, M. ) C. Wöhrmeyer et al.: New spinel containing calcium aluminate cement for corrosion resistant castables. Unitecr 2011, Kyoto, Japan, pp (2011). [7] P. Gehre et al.: Improved spinel-containing refractory castables for slagging gasifiers. Journal European Ceramic Society 33, pp (2013). [8] Y. Wen et al.: Effects of sintering temperature on pore characterization and strength of porous cordierte-mullite ceramics by a pore-forming in-situ technique. Int. Journal of Materials Research, 103 (10), pp (2012).